NETW 704
Signaling &
Network Control
ISDN User Part (ISUP)
Dr. Eng. Amr T. Abdel-Hamid
Winter 2006
Amr Talaat, 2006
ISUP
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Responsible for setting up and releasing trunks used for interexchange calls.
Created to provide core network signaling that is compatible with
ISDN access signaling.
Today, the use of ISUP in the network has far exceeded the use of
ISDN on the access side.
ISUP provides signaling for both non-ISDN and ISDN traffic; used by
basic telephone service phones.
The primary benefits of ISUP are
 speed, increased signaling bandwidth, and standardization of
message exchange. P
 Provides faster call setup times than Channel Associated
Signaling (CAS), it ultimately uses trunk resources more
effectively.
 Enables more call-related information to be exchanged.
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ISUP (cont.)
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Messages and parameters do vary between different countries, a
given variant provides a standard means of exchanging information
between vendor equipment within the national network, and to a
large degree, at the international level.
ISUP consists of call processing, supplementary services, and
maintenance functions.
Main components of ISUP:
 Bearers and Signaling
 ISUP Message Flow
 ISUP Message Format
 Message Timers
 Circuit Identification Codes
 Enbloc and Overlap Address Signaling
 Circuit Glare
 Continuity Test
 Interworking with ISDN
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Bearers and Signaling
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ISUP allows the call control signaling to be separated
from the circuit that carries the voice stream over
interoffice trunks.
The circuit that carries the voice portion of the call is
known within the telephone industry by many different
terms. Voice channel, voice circuit, trunk member,
and bearer.
If the signaling travels on a single linkset that originates
and terminates at the same nodes as the bearer circuit,
the signaling mode is associated.
If the signaling travels over two or more linksets and at
least one intermediate node, the signaling mode is
quasi-associated.
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ISUP Signaling Mode
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ISUP Signaling Mode
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The signaling mode used for ISUP depends greatly on
what SS7 network architecture is used.
For example, North America uses hierarchical STPs for
aggregation of signaling traffic. Therefore, most ISUP
trunks are signaled using quasi-associated signaling.
U.K. uses quasi-associated signaling for some SSPs,
they also heavily use associated signaling with directly
connected signaling links between many SSPs.
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ISUP Protocol
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A connection exists between ISUP and
both the SCCP and MTP3 levels.
ISUP uses the MTP3 transport services to
exchange network messages, such as
those used for call setup and clear down.
The "Interworking with ISDN" section of this
chapter further discusses end-to-end
signaling and the two different methods
using MTP3 and SCCP for transport.
Amr Talaat, 2006
ISUP Message Flow
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A core set of five to six messages represent the majority
of the ISUP traffic on most SS7 networks.
A basic call can be divided into three distinct phases:
 Setup
 Conversation (or data exchange for voice-band data
calls)
 Release
ISUP is primarily involved in the set-up and release
phases.
Further ISUP signaling can take place if a
supplementary service is invoked during the
conversation phase.
Amr Talaat, 2006
Messages
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A core set of five to six messages represent the majority
of the ISUP traffic on most SS7 networks. Yet, there are
more than 50 messages that are used in the ISUP
A basic call can be divided into three distinct phases:
 Setup
 Conversation (or data exchange for voice-band data
calls)
 Release
ISUP is primarily involved in the set-up and release
phases.
Further ISUP signaling can take place if a
supplementary service is invoked during the
conversation phase.
Amr Talaat, 2006
Message Timers
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ITU Q.764 defines the ISUP timers and their value ranges:
 T7 awaiting address complete timer: Also known as the network
protection timer. T7 is started when an IAM is sent, and is
canceled when an ACM is received.
 T8 awaiting continuity timer: Started when an IAM is received with
the Continuity Indicator bit set. The timer is stopped when the
Continuity Message is received.
 T9 awaiting answer timer: started when an ACM is received, and
is canceled when an ANM is received. If T9 expires, the circuit is
released.
 T1 release complete timer: T1 is started when a REL is sent and
canceled when a RLC is received. If T1 expires, REL is
retransmitted.
 T5 initial release complete timer: T5 is also started when a REL is
sent, and is canceled when a RLC is received. T5 is a longer
duration timer than T1 and is intended to provide a mechanism to
recover a nonresponding circuit for which a release has been
initiated. If T5 expires, a RSC is sent and REL is no longer sent
for the nonresponding circuit.
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Circuit Identification Codes
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The separation of signaling and voice create the need
for a means of associating the two entities.
ISUP uses a Circuit Identification Code (CIC) to identify
each voice circuit.
For example, each of the 24 channels of a T1 span (or
30 channels of an E1 span) has a CIC associated with
it. When ISUP messages are sent between nodes,
they always include the CIC to which they use.
Otherwise, the receiving end would have no way to
determine the circuit to which the incoming message
should be applied.
Because the CIC identifies a bearer circuit between
two nodes, the node at each end of the trunk must
define the same CIC for the same physical voice
channel.
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CIC
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CIC (cont.)
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ITU defines a 12-bit CIC, allowing up to 4096 circuits to be
defined. ANSI uses a larger CIC value of 14 bits, allowing for
up to 16,384 circuits.
An association must be created between the circuit and the
SS7 network destination.
This association is created through provisioning at the SSP, by
linking a trunk group to a routeset or DPC.
The CIC must be unique to each DPC that the SSP defines.
A CIC can be used again within the same SSP, as long as it is
not duplicated for the same DPC.
CIC 0 used many times throughout an SS7 network, and even
multiple times at the same SSP.
Unidentified Circuit Codes
 When a message is received with a CIC that is not defined
at the receiving node, an Unequipped Circuit Code (UCIC)
message is sent in response. The UCIC message's CIC
field contains the unidentified code. The UCIC message is
used only in national networks.
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CID/DPC
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Enbloc and Overlap Address Signaling
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When using ISUP to set up a call:
The Called Party Number (CdPN) can be sent using either
enbloc or overlap signaling.
In North America, enbloc signaling is used. Europe, both
methods are used.
Enbloc Signaling: The enbloc signaling method transmits the
number as a complete entity in a single message. When using
enbloc signaling, the complete number is sent in the IAM to set
up a call. Enbloc signaling is better suited for use where fixedlength dialing plans are used, such as in North America.
Overlap Signaling: Overlap signaling sends portions of the
number in separate messages as digits are collected from the
originator. Using overlap signaling, call setup can begin before
all the digits have been collected. When using the overlap
method, the IAM contains the first set of digits. The
Subsequent Address Message (SAM) is used to transport the
remaining digits.
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Enbloc
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Overlap
Signaling
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Overlap Signaling
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Overlap signaling is preferable because it decreases postdial delay. As shown in the preceding example, each
succeeding call leg is set up as soon as enough digits have
been collected to identify the next exchange.
overlap signaling is less efficient in terms of signaling
bandwidth.
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Circuit Glare (Dual-Seizure)
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Circuit Glare (Dual-Seizure)
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Resolving Glare
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When glare is detected, one node must back down and give
control to the other end. while the other call must be reattempted
on another CIC.
There are different methods for resolving which end takes
control. For normal 64-kb/s connections, two methods are
commonly used:
 the point code and CIC numbers are used to determine
which end takes control of the circuit. The node with the
higher-numbered point code takes control of even number
CICs, and the node with the lower-numbered point code
takes control of odd numbered CICs.
 prior agreement between the two nodes about which end will
back down.
 when glare occurs. One node is provisioned to always back
down, while the other node is provisioned to
Amr Talaat, 2006
Circuit Glare (Dual-Seizure)
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Avoiding Glare
 glare conditions can be minimized by properly
coordinating the trunk selection algorithms at each
end of a trunk group.
 A common method is to perform trunk selection in
ascending order of the trunk member number at one
end of the trunk group, and in descending order at the
other end.
 use the "Most Idle" trunk selection while the other end
uses the "Least Idle" selection.
 The idea is to have an SSP select a trunk that is least
likely to be selected by the SSP at the other end of
the trunk group.
Amr Talaat, 2006
ISUP Message Format
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The User Data portion of the MTP3 Signaling Information
Field contains the ISUP message, identified by a Service
Indicator of 5 in the MTP3 SIO field.
Each ISUP message follows a standard format that
includes the following information:
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CIC: The Circuit Identification Code for the circuit to which the
message is related.
Message Type: The ISUP Message Type for the message (for
example, an IAM, ACM, and so on).
Mandatory Fixed Part: Required message parameters that are of
fixed length.
Mandatory Variable Part: Required message parameters that are
of variable length. Each variable parameter has the following
form: Length of Parameter, Parameter Contents
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Because the parameter is not a fixed length, a
field is included to specify the actual length.
Optional Part: Optional fields that can be
included in the message, but are not mandatory.
Each optional parameter has the following form:
Parameter Name, Length of Parameter,
Parameter Contents
Figure 8-10 shows the ISUP message structure,
as described here.
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Local Number Portability (LNP)
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LNP was defined in the Telecommunications Act of 1996
as the
“ability of users of telecommunications services to retain,
at the same location, existing telecommunications
numbers without impairment of quality, reliability, or
convenience when switching from one
telecommunications carrier to another.”
The Telecommunications Act mandated that all
telecommunications service providers provide, to the
extent technically feasible, number portability in
accordance with the requirements prescribed by the
Commission.
Amr Talaat, 2006
LNP Specifications
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The following are some highlights from the FCC docket:
 The solution must support existing services and
features.
 LNP must use the existing numbering resources
efficiently.
 LNP cannot require subscribers to change their
telephone numbers.
 There can be no unreasonable degradation in service
(such as call setup delays) or network reliability
degradation when subscribers switch carriers.
 No carrier can have a proprietary interest.
 The LNP solution must be able to accommodate
location and service portability in the future.
 There can be no significant adverse impact outside
areas where number portability is deployed.
Amr Talaat, 2006
LNP Types
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There are three phases to LNP:
Service provider portability, enables a subscriber to
select a new local service provider while keeping his or
her existing telephone number. (Same Rate Center)
Service portability: This enables subscribers to change
the type of service they have while keeping their
telephone numbers. For example, if a subscriber
changes from a Plain Old Telephone Service (POTS) line
to an Integrated Services Digital Network (ISDN) service.
Location portability: enable a subscriber to move from
city to city, or even state to state, while maintaining the
same telephone number.
Amr Talaat, 2006
LNP Solutions
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There have been several proposals for providing LNP without
implementing a database:
 Call forwarding.
 Rejected because of the delay imposed on the calling party
while the carriers tried to route the call.
 Query-on-Release (QoR). When a call is routed to a number
that has been ported, the receiving switch identifies the number
as being vacant and returns an SS7 REL with an appropriate
cause code. The originating switch would then initiate a
database query to determine if the number had been ported.
This approach
 reduces the traffic across the SS7 network
 lessens the impact of the database queries
 places unnecessary delays on setting up telephone calls to
subscribers who have changed carriers.
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LRN
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The solution that was chosen was the LRN method. The
end-office switches in the rate center have a table
identifying all NPA-NXXs, which have numbers in them
that have been ported. The specific number is not
provided in the database, so the switch must initiate a
query if it is determined that the number dialed was to an
NPA-NXX considered as ported.